Composite material

ABSTRACT

To realize an improved plateau region stress level in a composite material that includes a superelastic shape memory alloy as a matrix, the composite material is a composite material including a superelastic shape memory alloy as a matrix, with carbon nanomaterials dispersed in the matrix.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of International ApplicationPCT/JP2011/074131 filed on Oct. 20, 2011, which claims priority toJapanese Patent Application No. 2010-245023 filed on Nov. 1, 2010, theentire content of both of which is hereby incorporated by reference.

TECHNICAL FIELD

The present invention generally relates to a composite materialcomprising a superelastic shape memory alloy as a matrix.

BACKGROUND DISCUSSION

NiTi alloys, FeMnSi alloys, and CuAlNi alloys are generally called shapememory alloys, and there is an alloy (superelastic shape memory alloy)showing superelasticity at least at human body temperature (around 37°C.). The term, “superelasticity” herein means the properties, of whicheven if the material is deformed (bent, stretched, compressed, andtwisted) at service temperature to the region in which ordinary metalsundergo plastic deformation, releasing the deformation results inrecovery to nearly the original shape before deformation withoutheating.

These characteristics of such superelastic shape memory alloys have beenused in various applications, and for example, NiTi alloys are used asthe base material for medical devices such as stents and guide wires(see the claims in Japanese Patent Application Laid-Open No. 2003-325655and paragraphs [0011] and [0016] in Japanese Patent ApplicationLaid-Open No. Hei 9-182799.

Since such superelastic shape memory alloys are generally a “softmetal”, in some cases, the stress in the plateau region (the region inwhich the stress remains nearly constant in an increase of strain in thestress-strain curve) is insufficient depending on the application.

SUMMARY

The present inventors extensively studied this matter and discoveredthat it is possible to improve the plateau region stress level byutilizing a composite material comprised of superelastic shape memoryalloy as a matrix, with carbon nanomaterials dispersed in the matrix.

According to one aspect of the disclosure here, a medical device is madeof composite material, wherein the composite material comprises a matrixthat includes a superelastic shape memory alloy and carbonnanomaterials.

According to another aspect, a composite material comprises asuperelastic shape memory alloy as a matrix, wherein carbonnanomaterials are dispersed in the matrix.

The content of the carbon nanomaterials can be 0.01 parts by mass to 0.5parts by mass relative to 100 parts by mass of the superelastic shapememory alloy. The superelastic shape memory alloy can be a NiTi alloy,and the matrix can be a sintered body of the superelastic shape memoryalloy. The carbon nanomaterials can be carbon nanotubes or carbon black.

The composite material disclosed here exhibits an improved plateauregion stress level.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating the results of tensile tests in Example 1and Comparative Example 1.

FIG. 2 is a graph illustrating the results of hysteresis tests inExample 1.

FIG. 3 is a graph illustrating the results of hysteresis tests inComparative Example 1.

FIG. 4 is a graph illustrating the results of tensile tests in Examples2 to 7 and Comparative Example 2.

FIG. 5 is a graph illustrating the results of cycle 1 of hysteresistests in Examples 2 to 7 and Comparative Example 2.

FIG. 6 is a graph illustrating the results of cycle 2 of the hysteresistests in Examples 2 to 7 and Comparative Example 2.

FIG. 7 is a graph illustrating the results of cycle 3 of the hysteresistests in Examples 2 to 7 and Comparative Example 2.

DETAILED DESCRIPTION

A composite material according to the disclosure here is a compositematerial comprising a superelastic shape memory alloy as a matrix, inwhich carbon nanomaterials are dispersed. Each component constitutingthe composite material according to the disclosure here is described indetail below.

<Matrix>

The matrix is derived from superelastic shape memory alloys, and is, forexample, a sintered body of the superelastic shape memory alloys.

Examples of the superelastic shape memory alloy herein include NiTialloys, CuAlNi alloys, FeMnSi alloys, CuSn alloys, CuZn alloys, InNiTiAlalloys, FePt alloys, and MnCu alloys. Among them, NiTi alloys arepreferred since they can recover from large strains and have excellentbiocompatibility.

Representative NiTi alloys include NiTi alloys containing 43% by weightto 57% by weight of Ni and the balance of Ti and unavoidable impurities.A small amount of other elements, for example, cobalt, iron, palladium,platinum, boron, aluminum, silicon, vanadium, niobium, or copper may beadded to such NiTi alloys. Among NiTi alloys, alloys containing 54.5% byweight to 57% by weight of Ni and the balance of Ti and unavoidableimpurities are particularly preferred. Such NiTi alloys may contain, inaddition to Ti and Ni, 0.070% by weight or less of C, 0.050% by weightor less of Co, 0.010% by weight or less of Cu, 0.010% by weight or lessof Cr, 0.005% by weight or less of H, 0.050% by weight or less of Fe,0.025% by weight or less of Nb, and 0.050% by weight or less of O.

<Carbon Nanomaterials>

The carbon nanomaterials are nanosized materials comprising carbonatoms. The composite material according to the disclosure here issuperior in stress in a plateau region relative to superelastic shapememory alloys alone due to the carbon nanomaterials dispersed in thematrix. That is, the plateau region for the composite material exists ata higher stress level than the plateau region for superelastic shapememory alloys alone. It is considered that the improvement is due toreinforcing the dispersed second phase and reinforcing the refinementwith the carbon nanomaterials (i.e., the carbon nanomaterial aredispersed in the superelastic shape memory alloy, and so the grain ofthe superelastic shape memory alloys is refined).

The carbon nanomaterials include carbon nanotubes (CNT), carbon black,fullerenes, and carbon nanocoils. Among them, carbon nanotubes andcarbon black are preferred because they can be mass-produced withconsistent high quality, and carbon nanotubes are more preferred becausetheir aspect ratio is high.

Carbon nanotubes include, for example, single-layered carbon nanotubes(SWCNT) and multi-layered carbon nanotubes (MWCNT).

The shape of carbon nanotubes is not particularly limited, but anaverage diameter of the carbon nanotubes in the cross-section ispreferably 1 nm to 1,000 nm, more preferably 5 nm to 500 nm. Also, anaverage total length of carbon nanotubes is preferably 0.1 μm to 1,000μm, more preferably 10 μm to 1,000 μm. An aspect ratio of carbonnanotubes is preferably 10 to 10,000, more preferably 150 to 1,000.

An average particle diameter of carbon black is preferably 40 nm to 120nm, more preferably 80 nm to 120 nm.

The content of the carbon nanomaterials is not particularly limited, butthe fed content of the carbon nanomaterials (the content of the carbonnanomaterials as raw material before sintering) is preferably 0.01 partsby mass to 0.5 parts by mass relative to 100 parts by mass of thesuperelastic shape memory alloys, more preferably 0.01 parts by mass to0.3 parts by mass.

When the content of the carbon nanomaterials is in this range, thestress in the plateau region can be significantly improved. That is, theplateau region for the composite material occurs at a higher stresslevel

<Production Method>

The method for producing the composite material disclosed here is notparticularly limited, and includes, for example, a method involvingsintering a mixture of raw materials comprising the superelastic shapememory alloys and the carbon nanomaterials, and a method involvingmixing a sintered product of the superelastic shape memory alloys withthe carbon nanomaterials.

As the method of sintering a mixture of raw materials comprising thesuperelastic shape memory alloys and the carbon nanomaterials, forexample, a wet process can preferably be used.

In the wet process, the carbon nanomaterials are dispersed in apredetermined liquid binder to yield a dispersed solution, with whichthe superelastic shape memory alloys are mixed, followed by heating themixture to dry and remove the binder, thereby yielding powder of thesuperelastic shape memory alloys, to the surface of which the carbonnanomaterials are attached. The powder is then sintered and extruded toyield a composite material according to the disclosure here.

The sintering conditions are not particularly limited, but the sinteringtemperature is preferably 700° C. to 1,200° C., more preferably 800° C.to 1,100° C. When the sintering temperature is kept in this range, theplateau region stress level can be significantly improved while theplateau is maintained.

Application of the composite material according to the present inventionis not particularly limited, but the composite material can be used, forexample, as a preferred base material for medical devices such asstents, guide wires, embolization coils, inferior vena cava filters, andwires for orthodontics.

EXAMPLES

Set forth next is a description of various examples utilizing thedisclosure here, but it is to be understood that the invention here isin no way limited to the examples.

Example 1

[Mixing of MWCNT with TiNi Alloys]

MWCNT was added to a binder containing water as a main component todisperse, to which NiTi alloy powder was then added such that the ratioby mass of NiTi alloy powder to MWCNT was 100:0.08. The mixture was thenheated at 600° C. to dry and remove the binder, yielding NiTi allowpowder, to the surface of which MWCNT was attached.

[Sintering]

The NiTi alloy powder, to the surface of which MWCNT was attached, wassintered according to the following conditions to yield a sintered body.

-   -   Temperature: 900° C.    -   Retention time: 30 minutes    -   Atmosphere: Vacuum    -   Pressure: 40 MPa    -   Rate of temperature elevation: 20° C./min

[Hot Extrusion Processing]

The sintered body obtained was subjected to hot extrusion processingaccording to the following conditions to yield an extruded product.

-   -   Preheating temperature: 1,050° C.    -   Pre-overheating time: 10 minutes    -   Extrusion ratio: 6    -   Ram speed: 6 mm/sec

Example 2

MWCNT was mixed with TiNi alloy powder under the same conditions asExample 1 except for addition of the NiTi alloy powder such that theratio by mass of NiTi alloy powder to MWCNT was 100:0.07. The otherprocess conditions were the same as those in Example 1.

Example 3

MWCNT was mixed with TiNi alloy powder under the same conditions asExample 1 except for addition of the NiTi alloy powder such that theratio by mass of NiTi alloy powder to MWCNT was 100:0.09. The otherprocess conditions were the same as those in Example 1.

Example 4

[Mixing of MWCNT with TiNi Alloys]

MWCNT was added to and mixed with NiTi alloy powder such that the ratioby mass of NiTi alloy powder to MWCNT was 100:0.05.

[Sintering]

The mixture obtained was sintered according to the following conditionsto yield a sintered body.

-   -   Temperature: 900° C.    -   Retention time: 30 minutes    -   Atmosphere: Vacuum    -   Pressure: 40 MPa

[Hot Extrusion Processing]

The sintered body obtained was subjected to hot extrusion processingaccording to the following conditions to yield an extruded product.

-   -   Preheating temperature: 1,100° C.    -   Pre-overheating time: 10 minutes    -   Extrusion ratio: 6    -   Ram speed: 6 mm/sec

Example 5

MWCNT was mixed with TiNi alloy powder under the same conditions asExample 4 except for addition of MWCNT such that the ratio by mass ofNiTi alloy powder to MWCNT was 100:0.10. The other process conditionswere the same as those in Example 4.

Example 6

MWCNT was mixed with TiNi alloy powder under the same conditions asExample 4 except for addition of MWCNT such that the ratio by mass ofNiTi alloy powder to MWCNT was 100:0.15. The other process conditionswere the same as those in Example 4.

Example 7

MWCNT was mixed with TiNi alloy powder under the same conditions asExample 4 except for addition of MWCNT such that the ratio by mass ofNiTi alloy powder to MWCNT was 100:0.25. The other process conditionswere the same as those in Example 4.

Comparative Example 1

Without mixing with carbon nanomaterials, NiTi alloy powder alone wassintered under the same conditions as Example 1. The other processconditions were the same as those in Example 1.

Comparative Example 2

Without mixing with carbon nanomaterials, NiTi alloy powder alone wassintered under the same conditions as Example 4. The other processconditions were the same as those in Example 4.

<Evaluation> [Tensile Test]

Tensile tests of the extruded products obtained in Examples 1 to 7 andComparative Examples 1 and 2 were performed at ambient temperature underthe following conditions (n=2). FIGS. 1 and 4 illustrate the results inExample 1 and Comparative Example 1, and the results in Examples 2 to 7and Comparative Example 2, respectively.

-   -   Shape of test piece: Round bar    -   Diameter of test piece: 3.5 mm    -   Length of test piece: 20 mm    -   Test speed: Strain rate 5×10⁻⁴ s⁻¹

It was found from the graphs illustrated in FIGS. 1 and 4 that thecomposite materials in Examples 1 to 7, in which carbon nanotubes weredispersed in the matrix derived from NiTi alloys, were improved in theplateau region stress level as compared to a sintered body of NiTialloys alone in Comparative Examples 1 and 2. That is, the stress at theplateau region of the stress-strain curve is higher (greater) ascompared to Comparative Examples 1 and 2.

[Hysteresis Test]

Hysteresis tests involving applying, as a cycle, a constant strainfollowed by releasing the stress to the extruded products obtained inExamples 1 to 7 and Comparative Examples 1 and 2 were performedaccording to the following conditions (n=1). The test includes threecycles, in which the strain applied to the test pieces was at 4% at thebeginning (cycle 1), then at 8.5% (10% in Comparative Example 1 and 8%in Examples 2 to 7 and Comparative Example 2) (cycle 2), and finally at15% (14% in Examples 2 to 7 and Comparative Example 2) (cycle 3). FIGS.2 and 3 illustrate the results in Example 1 and the results inComparative Example 1, respectively. FIGS. 5, 6, and 7 illustrate theresults of cycles 1, 2, and 3 in Examples 2 to 7 and Comparative Example2, respectively.

-   -   Shape of test piece: Round bar    -   Diameter of test piece: 3.5 mm    -   Length of test piece: 20 mm    -   Test speed: Strain rate 5×10⁻⁴ s⁻¹

It was found from the graphs illustrated in FIGS. 2 and 3 as well asFIGS. 5 to 7 that of the composite materials in Examples 1 to 7, inwhich carbon nanotubes were dispersed in the matrix derived from NiTialloys, after releasing the stress, the deforming strain was recoveredto some degree to the level similar to the sintered body of NiTi alloysalone in Comparative Examples 1 and 2. That is, in the compositematerials in Examples 1 to 7, after releasing the stress, the strain(deformation) returned to a level similar to that experienced by theNiTi alloys alone in Comparative Examples 1 and 2. Thus, the compositematerials in Examples 1 to 7 are able to be subjected to a higher degreeof stress and yet return to a level of strain similar to thatexperienced by the NiTi alloys alone in Comparative Examples 1 and 2. InExamples 6 and 7, test pieces were broken during cycle 3.

The detailed description above describes features and aspects of acomposite material disclosed here. The invention is not limited,however, to the precise embodiments and variations described andillustrated. Various changes, modifications and equivalents could beeffected by one skilled in the art without departing from the spirit andscope of the invention as defined in the appended claims. It isexpressly intended that all such changes, modifications and equivalentswhich fall within the scope of the claims are embraced by the claims.

What is claimed is:
 1. A medical device made of composite material, thecomposite material comprising a matrix that includes a superelasticshape memory alloy and carbon nanomaterials.
 2. The medical deviceaccording to claim 1, wherein the carbon nanomaterials is present in thematrix in an amount of 0.01 parts by mass to 0.5 parts by mass relativeto 100 parts by mass of the superelastic shape memory alloy.
 3. Themedical device according to claim 1, wherein the superelastic shapememory alloy is a NiTi alloy.
 4. The medical device according to claim1, wherein the matrix is a sintered body of the superelastic shapememory alloy.
 5. The medical device according to claim 1, wherein thecarbon nanomaterials are either carbon nanotubes or carbon black.
 6. Themedical device according to claim 1, wherein the carbon nanomaterialsare either single-layered carbon nanotubes or are multi-layered carbonnanotubes.
 7. The medical device according to claim 1, wherein themedical device is one of a stent, a guide wire, an embolization coil, aninferior vena cava filter or an orthodontic wire.
 8. A compositematerial comprising a superelastic shape memory alloy as a matrix, withcarbon nanomaterials dispersed in the matrix.
 9. The composite materialaccording to claim 8, wherein the carbon nanomaterials is present in thematrix in an amount of 0.01 parts by mass to 0.5 parts by mass relativeto 100 parts by mass of the superelastic shape memory alloy.
 10. Thecomposite material according to claim 8, wherein the matrix is asintered body of the superelastic shape memory alloy.
 11. The compositematerial according to claim 8, wherein the carbon nanomaterials arecarbon nanotubes or carbon black.
 12. The composite material accordingto claim 8, wherein the superelastic shape memory alloy is a NiTi alloy.13. The composite material according to claim 9, wherein the matrix is asintered body of the superelastic shape memory alloy.
 14. The compositematerial according to claim 9, wherein the carbon nanomaterials arecarbon nanotubes or carbon black.
 15. The composite material accordingto claim 9, wherein the superelastic shape memory alloy is a NiTi alloy.16. The composite material according to claim 10, wherein the carbonnanomaterials are carbon nanotubes or carbon black.
 17. The compositematerial according to claim 10, wherein the superelastic shape memoryalloy is a NiTi alloy.
 18. The composite material according to claim 11,wherein the superelastic shape memory alloy is a NiTi alloy.
 19. Thecomposite material according to claim 8, wherein the carbonnanomaterials are either single-layered carbon nanotubes or aremulti-layered nanotubes.